In the field of robotics and Numerically Controlled (NC) motion systems, a great amount of effort and research has been dedicated to modeling and characterizing said motion devices and motion systems in pursuit of accuracy enhancement. With industrial robots in particular, manufacturers and after-market companies have focused mainly on modeling the ‘as-built’ conditions of a robot. Usually, parameters in the kinematic model of the robot are adjusted based on the results of a one-time volumetric calibration of the robot in a variety of poses throughout its work envelope; typically, an external metrology device such as a laser tracker is employed to measure and compare the actual versus commanded pose (or simply position) of the robot over a distribution of locations. A further expansion of the aforementioned volumetric calibration of a robot includes similar methods that can be periodically performed on the manufacturing floor, but are not intended to be carried out during production. Such off-line calibration methods only provide a snap-shot in time of the robot's characteristics, and do not account for the degradation in accuracy due to wear or possible thermal changes that inevitably occur in between system calibrations.
The art is replete with various prior art laser tracking systems methods, which can locate a target in any of three to six degrees of freedom (DOF), thereby aligning robotic devices relative to the target to perform operations on a workpiece. These prior art systems and methods are taught by U.S. Pat. Nos. 4,412,121 to Kremers et al., U.S. Pat. No. 4,707,129 to Hashimoto et al., U.S. Pat. No. 4,714,339 to Lau et al., U.S. Pat. No. 4,792,228 to Haffner, U.S. Pat. No. 5,042,709 to Cina et al., U.S. Pat. No. 5,100,229 to Lundberg et al., U.S. Pat. No. 5,907,229 to Snell, and U.S. Pat. No. 6,400,452 to Maynard. The U.S. Pat. No. 4,714,339 to Lau et al., for example, teaches a three dimensional tracking system is a simplification of the five degree of freedom tracking system.
Still other accuracy enhancement methods involve in-line updating of the robot's kinematic model parameters, usually either via periodically presenting the robot end effector (in various poses) to fixed sensors that typically do not exist within the actual ‘work volume’ of the robot or via providing ‘enhanced’ encoder read-outs for the robot joints (or combination thereof). At least one from this class of methods does involve measuring the robot end effector position in the ‘work volume’ of the robot, but does not accomplish this during the robot's actual work cycle. All the aforementioned methods, whether intended to be ‘one-shot’ or periodically updated, are ultimately only predictive, and can be considered ‘passive’ with respect to truly knowing the current pose of the end effector.
Active (real-time) measurement of a robot's end effector via external metrology devices has long been investigated, and many commercial applications are currently being undertaken or have already been implemented. Laser trackers and laser radar units certainly possess the requisite accuracies to sufficiently guide/correct a robot for a variety of manufacturing processes, but are single line of sight (LOS) devices. In the case of laser trackers, they require time to ‘search’ for their corner cube targets. For laser radar, hemispherical targets are typically scanned. Each type of system is prohibitively expensive and slow for widespread application in real-time active robotic correction. 6-DOF generation using traditional corner cube reflectors requires either multiple laser trackers or, more commonly, measuring multiple corner cube targets on a robot's end of arm tool (EOAT). Many specialized target designs have been described that are meant to determine 5-DOF or 6-DOF of said target by employing a single line of sight from a laser tracker (using interferometric and/or time of flight techniques). Such a device employing a corner cube with an apical opening, smaller than the laser beam diameter, that allows a part of the beam to strike a photosensitive detector behind it, thereby providing 5-DOF (x,y,z,tilt,yaw) of the target is described in the U.S. Pat. No. 6,667,798 to Markendorf et al. United States Publication No. 20060222314 to Zumbrunn, et al., for example, adds patterned photo-emitters to the 5-DOF target; when measured by an external camera incorporated onto the laser tracker, target roll can also be determined. Commercially, the active probe possesses multiple LEDs that are picked up by the camera piggy-backed on the laser tracker. In this case, the laser tracker does not waste time searching for the corner cube since it uses a priori information of the probe's location via the camera's measurement of the LEDs. There are several limitations for solutions of this sort. Since the apex angles between the LEDs are quite small as viewed by the camera, any error in determining the transverse position of the LEDs contributes to angular errors in the 6-DOF solution. Similar angular ambiguity results from the distance between the photosensitive device and the corner cube being so small; any error in calculating the position of the laser spot on the photosensitive surface results in a large angular error of the target itself owing to the need for keeping the target's dimensions small enough to fit on a robot end effector. Additionally, this grey-scale option is quite expensive, and the probe is too large and directionally-limited to be mounted on many robotic end effectors, especially where the process requires that the robot exercises a full range of poses. United States Publication No. 20030043362 to Lau et al. describes an active target probe used in conjunction with a laser tracker that provides 6-DOF of the target, wherein polarized light is used to deduce roll of the target. This target also has the advantage of rotating to keep its aperture perpendicular to the incident laser tracker beam. Still, this target has angular limitations for yaw, pitch, and roll detection; lacks the requisite accuracy for higher-precision robot correction applications; is still too large to incorporate into many end-effector designs; and is expensive. The probes described here are generally too large to be positioned close to the tool center point (TCP) of the robot, resulting in ‘lever-arm’ effects when ascertaining the pose of the TCP. And, coupled with the fact that they require a laser tracker or camera-enhanced laser tracker to perform, such systems are prohibitively expensive, especially when compared to the base price of a standard articulated arm robot.
Indoor optical GPS has recently made inroads to many manufacturing solutions and can provide the current pose of a robot, but such systems cannot at this time demonstrate accuracies near those that are needed for high precision robot guidance applications. The systems do have receivers with large fields of view over which to pick up the laser output of the transmitters, but are still LOS devices. In the context of high precision robot guidance, the cost-effectiveness of indoor GPS can only be realized when large numbers of receivers are required on the manufacturing floor.
Photogrammetry has been employed for active robotic correction to varying degrees of success. Most use end effector-mounted ‘active’ targets, such as LEDs, and do not depend on more traditional techniques using external lighting of reflective stickers. These photogrammetric solutions generally fall into two categories. The first involves ‘single sensor housing’ solutions, where multiple photosensitive devices are distributed within a single housing (typically there are three distinct LOS emanating from the housing). The second involves using multiple, statically-positioned cameras whose fields of view provide overlap within the volume of interest. Photogrammetric solutions have the great advantage of very high refresh rates (for example, three targets can typically be measured in less than a millisecond, thus providing 6-DOF of an object). This speed allows for dynamic tracking of multiple coordinate frames, and can even tolerate most production environment vibrations. Considering these features, one would logically conclude that this class of solutions holds the most promise for high precision active robotic correction. There are a few subtleties that bear explanation, however. First off, the volume of interest of ‘single sensor housing’ photogrammetric solutions is limited to a wedge that typically extends only to 6 meters from the device (the closer you get to the sensor, the smaller the transverse field of view becomes). Since the allowable spacing between the LED targets that need to be placed on a typical robot end effector is usually small, poor apex angles generally result between the LEDs as viewed by the system. In an effort to put all the photosensitive devices and optics in a single sensor housing, the apex angles between each line of sight are likewise compromised. Thus, while these types of sensors are typically quite accurate in the transverse directions, distance determination is the weakest component of the 6-DOF transform. The poor apex angles could be corrected by adding another photogrammetric head in the work cell at nearly 90 degrees to the first photogrammetric head, but the resulting overlap between the two wedge volumes becomes prohibitively small for most applications. Taking into consideration that a single head photogrammetric system typically costs as much as a laser tracker, the cost per measurement volume becomes a huge factor. The second approach to active target photogrammetry generally uses multiple sensors with overlapping fields of view, achieving significantly better apex angles among the cameras. During operation, the sensors are statically positioned, and the cameras must be aggressively internally calibrated over their entire volumes of interest. Again, though, the volume of overlap between the cameras is limited. And, while the sensors for these types of photogrammetric systems are cheaper than the ‘single sensor housing’ varieties, they are still considerable when compared to the cost of a robot, so adding additional LOS capability by adding more sensors is seldom a viable option.
Still another class of devices that could be used for determining the pose of a robot EOAT includes theodolites and total stations. There are now total station models that are automated, allowing the electronic theodolite to be aimed/driven remotely via a computer. These devices also include time of flight ranging devices that employ reflective targets (allowing for line of sight measurements up to a few kilometers) or no targets at all (lighter-colored surfaces can be measured at a few hundred meters' distance). Ranging accuracy is typically on the order of 2-3 mm. Pointing accuracy (azimuth, elevation) range from 5-8 arcseconds for construction grade systems all the way up to 0.5 arcseconds in precision systems. As a stand-alone gimbal, such systems cannot provide accuracies greater than those already achieved by robots with enhanced accuracy modeling. Even if the ranging capability was not used in favor of locating the angular positions of 3+ reflectors on a robot EOAT and solving 6-DOF via traditional photogrammetric techniques, one again arrives at a poor apex-angle solution. Multiple gimbals would allow for photogrammetric determination of an EOAT's 6-DOF pose by allowing for more optimum apex angles, and the novel invention described herein seeks to do so by employing an even less-expensive alternative by obviating any range-detection hardware.
The inherent limitations of ‘passive’ robot correction, along with the performance shortcomings and cost barriers of existing ‘active’ robot correction systems, were all taken in to consideration when developing the following affordable, external active robot correction system. Additional techniques include multiple length measurement with laser, acoustics, or wires; and multiple camera-like systems. Stereo-triangulation is undesirable since it requires a minimum of two tracking systems and it is a static measuring technique. Similarly, imaging by camera is undesirable since the resolution of the system is typically much too low to adequately cover the working envelope of a robot, in addition to the unrealistic stabilities and accuracies required when generating/maintaining the internal calibrations of such optical systems.
Therefore, there is a long-standing need for an improved system and method of external robotic accuracy enhancement.